1. Field of the Invention
The present invention relates to a laser apparatus, an exposure head, and an exposure apparatus, and particularly relates to a suitable high-brightness laser apparatus which illuminates a spatial light-modulation element, an exposure head which exposes a photosensitive material with a laser beam modulated by a spatial light-modulation element in accordance with image data, and an exposure apparatus equipped with this exposure head.
Furthermore, the present invention relates to an optical fiber connection method and, in particular detail, to a method for connecting two optical fibers whose cladding diameters differ.
2. Description of the Prior Art
Heretofore, various exposure apparatuses which employ spatial light modulation elements such as digital micromirror devices (DMD) have been proposed for carrying out image exposure with light beams modulated in accordance with image data. An example of a DMD is a mirror device in which numerous micromirrors, which alter angles of reflection surfaces thereof in accordance with control signals, are arranged in a two-dimensional pattern on a semiconductor support of silicon or the like. An example of an exposure apparatus that utilizes such a DMD is, as shown in FIG. 15A, structured by a light source 1 which irradiates laser light, a lens system 2 which collimates the laser light irradiated from the light source 1, a DMD 3 which is disposed substantially at a focusing position of the lens system 2, and lens systems 4 and 6 which focus the laser light that has been reflected at the DMD 3 onto a scanning surface 5. In this exposure apparatus, the respective micromirrors of the DMD 3 are switched on and off by a control apparatus (not shown), in accordance with control signals generated in accordance with image data or the like, and modulate the laser light. Thus, image exposure is carried out by modulated laser light.
At the light source 1, a structural unit is plurally disposed. This structural unit is equipped, as shown in FIG. 29, with a single semiconductor laser 7, a single multi-mode optical fiber 8, and a pair of collimator lenses 9. The pair of collimator lenses 9 focuses laser light irradiated from the semiconductor laser 7 at an end face of the multi-mode optical fiber 8. The light source 1 is structured at a bundle-form fiber light source in which a plurality of the multi-mode optical fibers 8 are bundled.
Commonly, a laser with an output of around 30 mW (milliwatts) is employed as the semiconductor laser 7, and an optical fiber with a core diameter of 50 μm, a cladding diameter of 125 μm and an NA (numerical aperture) of 0.2 is employed as the multi-mode optical fiber 8. Accordingly, if an output of around 1 W (watt) is to be obtained, it is necessary to bundle a total of forty-eight (eight by six) of the multi-mode optical fibers 8 of the above-described structural units, and a diameter of the light emission point is about 1 mm.
However, in a conventional light source, the diameter of a light emission point becomes larger in the event of bundling and, consequently, there is a problem in that sufficient depth of focus cannot be obtained when a high resolution exposure head is to be structured. Sufficient focal depth cannot be obtained, in particular, in the case of very high resolution exposure with a beam diameter of around 1 μm. Moreover, beam blurring occurs at peripheral edge portions of an area-form exposure beam.
Further, when a high power output exposure head is structured with a conventional exposure head, the number of optical fibers that are bundled increases. Thus, there are problems in that, not only do costs increase, but diameter of a light emission point becomes larger, and the illumination NA with respect to the spatial light modulation element also becomes larger. As a result, the image formation NA of the image formation beam becomes larger. Thus, focal depth becomes shallower.